1. Field of the Invention
This invention relates to an integrated circuit device used for driving light emitting elements for use in an optical printer, an image sensor, etc., and heating elements of a thermal head, and more particularly to an integrated circuit device effective when a drive current of driven elements is adjusted.
2. Discussion of the Related Art
For example, an optical printer head includes a linear array of a number of LEDs (light emitting diodes) as light emitting elements on a printed wiring board and is driven by several drive ICs (drive integrated circuits). One of the drive ICs drives, for example, 64 LEDs and therefore contains a shift register and a latch circuit each having bit cells corresponding to the 64 LEDs and as many logic gates and drive MOSFETs (metal oxide semiconductor field effect transistors) as the number of LEDs. In the LED drive IC of this type, drive current of the drive MOSFET, namely, current flowing through the LED varies due to variation in supplied gate potential, IC characteristics, etc., and the light emitting amounts of the LEDs differ from each other, lowering the print quality. To avoid this disadvantage, the supply potential to each drive MOSFET gate is adjusted for each IC. A D/A converter is used as a conventional method for adjusting the gate potential.
FIG. 1 shows a specific circuit therefor, wherein an output signal of an n-bit D/A converter 51 is applied via a logic gate circuit 52 to a gate of a MOSFET 54 for driving an LED 53. The D/A converter 51 outputs an analog voltage V.sub.0 corresponding to a digital setting in the range of 0 V to the supply voltage V.sub.DD. When the logical gate circuit 52 is enabled, the output voltage V.sub.0 is applied to the gate of the MOSFET 54 to allow a drive current I.sub.b corresponding to the voltage V.sub.0 to flow.
The above-mentioned conventional circuit device uses the D/A converter for adjusting the potential supplied to the gate of the drive MOSFET, and the power supply voltage supplied to the D/A converter is also V.sub.DD as a matter of course. By the way, when a large number of driven elements are driven at a time, a large drive current flows, thus the supply voltage V.sub.DD often fluctuates. As the supply voltage V.sub.DD fluctuates, the source-to-drain current of the drive MOSFET also fluctuates. Thus, the drive current of the driven element is not stabilized and fluctuates in response to fluctuation of the supply voltage.